Hindered nitroxides based on triacetoneamine, 4-oxo-2,2,6,6-tetramethylpiperidine have been known and two Nitroxides, 4-oxo TEMPO and 4-Hydroxy TEMPO have been used commercially for many years. These 4-substituted 1-oxyl-2,2,6,6-tetramethylpiperidines, generally produced in situ by oxidation of the parent amine, are used in polypropylene to prevent UV light degradation of the plastic. A good source of information of these uses is Oxidation Inhibition in Organic Materials, Volume II by Jan Pospisil and Peter P. Klemchuk, CRC Press, Inc., Boca Raton, Fla., 1990). When used in oxygen free atmospheres, the nitroxide itself is necessary to inhibit spurious free radical reactions. Consequently the pure nitroxyl compounds have been synthesized and offered commercially to inhibit polymerization of unsaturated hydrocarbon, styrene, vinyl and acrylic monomers during processing, distillation and storage. Hindered Nitroxides are also used as catalysts in selective oxidation of alcohols and experimentally for living polymerization.
4-Oxo-2,2,6,6-tetramethylpiperidine also known as 2,2,6,6-tetramethyl-4-piperidone or triacetoneamine, as produced commercially, is oxidized to produce 1-oxyl-4-oxo-2,2,6,6-tetramethylpiperidine, also known as 4-oxo TEMPO, a nitroxide with poor stability, even at ambient temperatures. This nitroxide has achieved limited use due to its poor stability even though its cost is low.
4-Hydroxy-2,2,6,6-tetramethylpiperidine is produced commercially by hydrogenation of 4-oxo 2,2,6,6-tetramethylpiperidine. 1-Oxyl-4-hydroxy-2,2,6,6-tetramethylpiperidine, also known as 4-Hydroxy TEMPO, is produced commercially by oxidation of 4-hydroxy-2,2,6,6-tetramethylpiperidine. It is used to prevent undesired free radical polymerization in styrene production and in refinery streams. This use is described in U.S. Pat. No. 5,254,760 and in Soviet Patent No. 1,558,888. A brief review of this subject is found in U.S. Pat. No. 6,117,276. Its use is limited because of its poor hydrocarbon solubility and poor stability in oxidizing media. In addition, the relationship of the vapor pressure vs. temperature described in the publication Hydrogenation of Alkenes and Alkynes Di-tert-alkyl Amines Di-tert-alkyl Nitroxyl (Aminoxyl) Radicals Polymerization Inhibitors G. Sosnovsky, M. Jawdosiuk, and J. M. Clumpner, Zeitschrift für Naturforschung, 55, 109 (2000) indicates a low volatility of this inhibitor, thus diminishing it role for potential use in the vapor phase.
2,2,6,6-Tetramethylpiperidine is made from 4-oxo-2,2,6,6-tetramethylpiperidine by the Wolff-Kishner reduction with hydrazine hydrate in ethylene or diethylene glycol. It is used to produce the nitroxide, 1-oxyl-2,2,6,6-tetramethylpiperidine, also known as TEMPO. It is used primarily to prevent undesired free radical polymerization. It is also used experimentally for living polymerization and for selective oxidation. While this nitroxide has good thermal stability, it has limited use because of the toxicity of the amine and its high cost.
2,2,6,6-Tetramethylpiperidine based spiro-ketal nitroxides have been mentioned in the literature but their physical properties have not been described there is no commercial supply available. A recent description of spiro-ketal nitroxyl radicals appeared in German Patent 42 19 471 A1 “N-Oxyl Derivatives of 2,2,6,6-Tetramethylpiperidine and Their Preparation” filed on Jun. 13, 1992. This patent relates to the preparation of nitroxyl radicals by catalytic oxidation of 2,2,6,6-tetramethylpiperidine ketals obtained from the following alcohols and glycols: methanol, ethanol, propanol, isobutanol, and n-butanol, ethylene glycol, 1,2-propanediol (propylene glycol), and 2,2-dimethyl-1,3-propanediol (neopentyl glycol). The latter three glycols, when reacted with 2,2,6,6-tetramethylpiperidine and then oxidized with hydrogen peroxide in the presence of catalytic amounts of divalent metals, such as magnesium or zinc, generate spiro-ketal nitroxides. The patent does not describe any physical properties or practical use of these nitroxyl radicals. U.S. Pat. No. 3,790,525 “4-Piperidone Ketal Derivatives, Their Preparation And Their Use As Stabilizers”, filed Jan. 19, 1972 mentions a spiro-ketal nitroxide prepared from 1,3-propanediol and triacetoneamine, to form the ketal amine. No mention is made of the Nitroxide.
In U.S. Pat. No. 5,631,366 (May 20, 1997) 7,7,9,9-tetramethyl-1,4-dioxa-8-azaspiro[4.5]dec-8-yloxy, the spiro-ketal nitroxide derived from the reaction of ethylene glycol with 4-oxo-2,2,6,6-tetramethylpiperidine followed by oxidation, was used to convert an alcohol to an aldehyde is disclosed.
In JP 04362632 A2 (Dec. 15, 1992) photochromic materials containing 7,7,9,9-tetramethyl-1,4-dioxa-8-azaspiro[4.5]dec-8-yloxy or 2-(hydroxymethyl)-7,7,9,9-tetramethyl-1,4-dioxa-8-azaspiro[4.5]dec-8-yloxy spiro-ketal nitroxides were shown to have improved light resistance, transparency and evenness.
Yoshikawa and Negishi in JP 11286634 A2 (Oct. 19, 1999) have used 2-(hydroxymethyl)-7,7,9,9-tetramethyl-1,4-dioxa-8-azaspiro[4.5]dec-8-yloxy and similar spiro-ketal nitroxides, in thermosetting polymeric coating compositions of superior light stability.
The spiro-ketal nitroxides resulting from commercially available 1,3-propanediol and 2-methyl-1,3-propanediol were not mentioned in any of the above prior art.
We have now found that by oxidizing the ketals produced from 4-oxo-2,2,6,6-tetramethylpiperidine and 1,3-propanediol or 2-monosubstituted 1,3-propanediols with hydrogen peroxide, unique spiro-ketal nitroxides result. These unique spiro-ketal nitroxides have unique and useful properties displaying excellent (a) high-temperature stability, b) good volatility, (c) oxidative stability, (d) hydrophobic character (e) solubility in hydrocarbons and monomers and (f) inhibition of free radical reactions. They have unexpected advantages, when compared with commercially available nitroxides, in solubility in hydrocarbons and monomers, especially ethylbenzene and styrene. In addition, their high thermal stability and volatility makes it possible to use them in the vapor phase at elevated temperature employed in the commercial production of monomers. These are critical properties in the two major commercial use areas for nitroxides, styrene processing and refinery inhibitors.
The Invention